A ChartCalendar value, expressed in milliseconds, is the proper numeric value to use, in cases where there is not an explicit contructor, or method which accepts ChartCalendar values.
ChartCalendar []x1= new ChartCalendar[nnumpnts];
double []y1 = new double[nnumpnts];
.
.
.
// Place the vertical marker at data point x1[10]
Marker amark = new Marker(pTransform1, ChartObj.MARKER_VLINE, x1[10].GetCalendarMsecs(), 0, 2, ChartObj.PHYS_POS);
chartVu.AddChartObject(amark);